1. Field of the Invention
The invention relates to a steering device for steering the wheels of a steering axle of a motor vehicle having at least three axles, with the steering axle being designed as a trailing or leading axle.
2. Description of the Prior Development
In order to improve the driving behavior, for example in terms of manoeuvrability and tire wear, in a motor vehicle, particularly in a commercial vehicle or heavy-goods vehicle, with more than two axles, at least one rigid axle designed as a trailing or leading axle and having non-steerable wheels is replaced by a steering axle having steerable wheels. To steer the wheels of this trailing or leading axle, the vehicle is equipped with a steering device which is designated as additional steering. In this case, the hydraulics and electronics of the steering device cause these wheels to be steered as a function of the steering movements triggered by the driver, with the geometric arrangement of these wheels on the vehicle being taken into account.
In the event that such additional steering has malfunctions, advanced additional steerings of this type can change-over from normal operation with a fully functionable steering device to emergency operation, so that, in particular, the safety of the vehicle can be ensured. In this context, two different safety concepts have emerged in practice. According to one safety concept, the wheels steerable by means of the additional steering are locked in their instantaneous position, in which they are located at the moment of changeover to emergency operation. Alternatively, the wheels can be moved back by means of corresponding additional elements into a centered middle position assigned to the straight-line travel of the motor vehicle and only then are locked in this position. In the last-mentioned case, in emergency operation the wheels of the steering axle which are steerable per se function in the same way as non-steerable wheels of a rigid axle.
According to the other safety concept, the wheels of the steering axle which are steered by the steering device are cleared hydraulically in emergency operation, the mounting of the wheels or the design of the steering axle being such that these wheels, utilizing a castor effect, function in a self-tracking manner, at least when the vehicle is travelling forwards. In this case, the carrying capacity of the wheels is not impaired, but they can no longer transmit lateral guidance forces to the motor vehicle. The present invention relates to a steering device for wheels of this type, which are self-tracking in emergency operation.
To implement the hydraulic clearance of the steered wheels in emergency operation, a hydraulic arrangement has a clearing valve which is capable of being actuated by an electronic arrangement and by means of which the hydraulic connections of the actuating assembly are short-circuited in emergency operation, with result that the same hydraulic pressure prevails at both hydraulic connections and free fluid exchange is possible between these.
The present invention is concerned with the problem of improving a steering device of the type mentioned above, to the effect that vehicle safety is increased.
This problem is solved, according to the invention.
The arrangement of throttle elements, which is proposed according to the invention, gives rise to the situation where, in emergency operation, that is to say when the hydraulic connections of the actuating assembly communicate with one another hydraulically via the clearing valve, an exchange of hydraulic medium between these hydraulic connections can take place only in a throttled manner. When, for example, the vehicle travels on one side over an obstacle or unevenness in the ground, in the case of a conventional steering device the lateral forces occurring at the same time and acting on the cleared, that is to say laterally freely movable, wheels can bring about a brief deflection of the respective wheels, which may lead to an unsteady castor, in particular to wobbling of the respective cleared wheels. In contrast to this, in the steering device according to the invention, a steering movement of the cleared wheels which is generated by external forces is damped, since the fluid exchange between the hydraulic connections of the actuating assembly takes place in a throttled manner. By virtue of this measure, the cleared wheels have considerably improved caster properties and thereby contribute to increased vehicle safety in emergency operation.
Moreover, the measures proposed according to the invention afford an improvement in vehicle safety in normal operation. This is because, in known steering devices, the regulating/control valve is usually designed as a proportional valve which, depending on modulation or activation, transmits the operating pressure of a pressure generator connected to one of the connections of the proportional valve, more or less throttled, to one or other connection of the hydraulic actuating assembly. So that a sufficiently high actuating force and actuating speed can be ensured for the actuating assembly when the vehicle has a maximum load, the proportional valve is equipped with passage cross sections sufficiently large for these instances or throttle resistances sufficiently small for these instances. For the predominant majority of operating situations, however, only markedly lower actuating forces and/or actuating speeds have to be implemented by means of the hydraulic actuating assembly, so that, in these cases, the proportional valve has to be modulated or activated with small opening cross sections or with high throttle resistances within a small range only. However, this reduced modulation at the same time signifies considerably restricted resolution. That is to say the setting, made by the proportional valve, of the actuating speed and actuating travel and therefore of the steering speed and steering angle of the respective wheels can be set and regulated only relatively approximately. Tests have shown that the proportional valves designed for a maximum vehicle load and maximum steering speeds are, in practice, modulated only up to about 20% in the predominant majority of operating situations which arise.
With the aid of the throttles arranged according to the invention, however, it is possible, in order to implement the desired movements of the actuating member, to open the proportional valve further or to modulate it within a considerably wider range, since, to implement the same actuating speeds, the proportional valve must be opened or modulated to a greater extent because of the throttle resistances arranged in the hydraulic lines. The desired movement of the actuating member can therefore be controlled with greater resolution and consequently more accurately in terms of speed and travel.
In a preferred embodiment, the hydraulic arrangement may be designed as a closed-center arrangement in which the regulating/control valve shuts off a regulating/control valve connection connected to a pressure source, as long as there is no control deviation detected between the desired and the actual steering angle. This measure ensures that, when the vehicle is travelling in a straight line, in normal operation the steerable wheels are blocked hydraulically and have particularly good straight-line running properties.
Preferably, an embodiment of this type with a closed-center arrangement has a pressure accumulator, which is fed by the hydraulic pump according to requirements. An arrangement of this type has proved particularly effective in practice, since, for example, a considerably smaller hydraulic pump can be used.
However, in conventional steering devices, the advantages of a closed-center arrangement with a pressure accumulator may have an adverse effect on the additional steering in an emergency. For example, the regulating/control valve may, as it happens, be blocked by a malfunction in just such a position in which the pressure accumulator is connected to one of the hydraulic connections of the actuating assembly. Even when the electronics of the steering device switch off the hydraulic pump immediately, there are flows of hydraulic medium for at least a short time since the pressure accumulator expands due to the (partially) open regulating/control valve. Although the hydraulic connections of the actuating assembly are short-circuited by the clearing valve, during this discharge of the pressure accumulator a pressure difference may be formed at the connections of the clearing valve on account of the throttle losses at the regulating/control valve and at the clearing valve, this pressure difference being transmitted to the hydraulic connections of the actuating assembly and, in spite of clearance, generating at the latter an actuating force and therefore a steering force on the respective wheel. A brief deflection during changeover to emergency operation may thereby occur due to the discharge of the pressure accumulator, and, in conventional additional steerings this may lead to driving situations which cause problems.
In contrast to this, in the steering device according to the invention, because of the proposed arrangement of throttle elements the discharge of the pressure accumulator is throttled by these throttle elements. As result, the flow velocities are reduced and, consequently, the throttle effects during flow through the regulating/control valve and the clearing valve are reduced. The differential pressure which builds up between the connections of the clearing valve is thereby considerably lower and, moreover, again because of the throttle elements, can spread to the connections of the actuating assembly in a throttle manner only.
The measures proposed according to the invention can thus improve vehicle safety in many respects.